1. Field of the Invention
The present invention relates to a wire for a coil, having a square shape in cross section.
2. Related Art
It is known that most of conventional coil wires have a circular shape in section (hereinbelow, a coil wire having a circular sectional shape will be called a “round wire”). By covering a round wire as a conductor with an insulating layer, a round electric wire is formed. When a coil is manufactured by using such a round wire, naturally, a gap is created between round wires. The coil therefore has a drawback of a low packing factor. It is also known that a coil wire having a conductor whose sectional shape is a square is manufactured to solve the drawback (hereinbelow, a coil wire having a square shape in section will be called “square wire”).
However, it is also known that the square wire requires a know-how different from that of the conventional method of forming a round wire with respect to formation of an insulating layer and a winding method for obtaining a coil shape.
For example, the square wire has a drawback that it is difficult to form a uniform insulating layer at four corners in section. We have already proposed a method providing a novel insulating layer, and so on, to solve the drawback, thereby obtaining the insulating layer for which the shape of each corner is stable. We have found that the method produces an effect of improving mass productivity.
Generally, to improve a packing factor, an ideal square is preferable as a sectional shape. Specifically, a shape in which chamfers (including arc-shaped chamfers (“R part”) and linear chamfers) are not provided at all at the corners is preferable. In the case where the sectional shape is a perfect square, a wire is wound while sides are closely attached to each other, so that the sides function as a guide.
However, in the case of providing an insulating layer for a perfect square, which is not chamfered, a problem occurs such that the thickness of the insulating layer at the corners varies. For example, in a wire generally called a slit wire obtained by cutting a thin plate made of a conductive material and having a predetermined thickness into parts each having a predetermined width by slitter, the corners are not chamfered, but a small burr which is disadvantageous to form an insulating film occurs. In the case of providing an insulating layer for a square wire as the slit line, the thickness of the insulating layer in the corner varies, as mentioned above. Naturally, it is not preferable as a coil wire. In addition, in the case of providing an insulating layer, in a square wire having no chamfers at corners, a phenomenon tends to occur that the insulating layer is destroyed in a position at which a layer lies on another layer in a winding process, for example, in a position at which the second layer lies on the first layer.
Since the corners of the square wire are not chamfered, the corners easily come into engagement with each other. On the other hand, in the case where the corners are chamfered, if the dimension of the chamfer is too large, a so-called rolling phenomenon occurs in the winding process. As a result, the packing factor becomes lower than that of the round wire, and the performance deteriorates.